1. Field of the Invention
This invention relates to oxygen-rich gas breathing systems and is more particularly concerned with a system and apparatus for providing oxygen-rich gas for breathing by aircrew and passengers of commercial passenger aircraft and for use in medical evacuation aircraft.
2. Description of the Prior Art
It is a requirement in passenger carrying aircraft that oxygen be provided for breathing by flight crew, cabin crew and passengers, particularly in emergency situations such as cabin decompression. Generally the flight crew breathing system is separate from the passenger breathing system and oxygen is supplied from pressurised gaseous oxygen cylinders. It is also a requirement that oxygen be available for continuous breathing by one of the aircrew flying the aircraft during flight above 12000 meters to provide protection against the effect of altitude in the event of instantaneous cabin decompression. Ideally oxygen for the latter requirement should be made available without depleting the stored oxygen source for the emergency system. The passenger system is designed to provide protection against cabin decompression at altitude and is required to provide oxygen for up to ten minutes duration while the aircraft descends to a safe altitude of 3000 meters. For this purpose chlorate candles are generally used although gaseous oxygen may also be made available in the passenger cabin for use by the cabin crew for therapeutic purposes during high altitude flight. In some larger aircraft gaseous oxygen may be provided for the passenger system and additional gaseous oxygen may be carried for extended duration oxygen use over the ten minutes requirement.
The development of new air routes in certain areas of the world presents a problem because some parts of these routes are over mountainous terrain which does not permit descent to 3000 meters in the event of cabin decompression. This gives rise to a requirement for oxygen to be made available for use at altitudes up to 6000 meters over longer periods which may be forty minutes or more. For durations above ten minutes, chlorate candle operation is unreliable and the provision of additional cylinders for storage of gaseous oxygen gives rise to weight and space penalties.
Systems using gaseous oxygen stored in cylinders require frequent servicing and recharging to maintain pressure, such servicing and re-charging being accomplished either on the aircraft from a ground source or by replacement of the cylinders. As the cylinders may have to be transportable outside of the aircraft they are subject to regulations which increase their weight compared to normal aircraft design practice.
U.S. Pat. No. 4,960,119 (Normalair-Garrett) discloses an aircraft aircrew life support system for use in military aircraft. The system comprises an on-board oxygen generating system (OBOGS) having molecular sieve adsorber beds or oxygen concentrators controlled to deliver breathable gas enriched with oxygen to a percentage concentration, say 55% to 60%, suitable for meeting aircrew breathing requirements during normal flight operation from ground level to maximum aircraft operating altitude of 18300 meters (60000 feet) with a pressurised cabin, i.e. maximum aircraft cabin operating altitude of 6100 meters (20000 feet). Breathable gas in this concentration range is increased in pressure by a pressure intensifier or compressor before being delivered to a storage tank from which it is supplied to an aircrew member by way of a breathing regulator and a face mask. Additionally, at times when the pressure in the storage tank is above a predetermined value and it is not being charged, the OBOGS may be switched to control the oxygen concentrators to deliver breathable gas of maximum oxygen concentration, at least 90% oxygen, for topping up an emergency breathable gas storage bottle associated with an ejection seat and for use during bail out, if the pressure in this storage bottle is sensed to have fallen below a predetermined minimum value. Because of the need to supply breathable gas in all flight regimes, the oxygen concentrators and the compressor must be sized to meet this requirement and the storage tank must have a capacity to handle the breathing gas demand including peaks in breathing demand when the aircrew member is breathing deeply or fast during a strenuous flight manoeuvre. At the same time provision must be made for changing cycling of the oxygen concentrators to obtain delivery of substantially pure oxygen for meeting the requirement to top up the emergency gas storage bottle.
Other disclosures of military aircraft breathing systems having a facility for storing breathable gas delivered by an OBOGS are to be found in U.S. Pat. No. 4,428,372 (issued to Linde and now assigned to present applicant) and U.S. Pat. No. 4,651,728 (Boeing). In U.S. Pat. No. 4,428,372 a portion of the oxygen-rich breathing gas delivered by oxygen concentrators is stored in an emergency storage tank from which it may be withdrawn for breathing in the event of interruption of the normal supply from the concentrators. U.S. Pat. No. 4,651,728 provides a system having a standby supply of breathable gas and an emergency supply of breathable gas stored in separate cylinders. A selector valve is provided whereby breathable gas may be withdrawn from the standby cylinder when the aircraft is on the ground with the OBOGS non-operational or during flight if the system malfunctions. The emergency gas cylinder is included primarily as a source of breathable gas for use by the aircrew during bail out from the aircraft but may also be used in the event of failure of both the OBOGS and the standby cylinder. Again, the systems which are the subject of these disclosures must meet the requirement for continuous breathing during normal flight operations from ground level to maximum aircraft operating altitude of an aircraft having one or two aircrew so that they must be sized accordingly.